The 6-litre capacity makes the Gravity ideal for small groups or basecamp use (a 10-litre version is also available for bigger groups). Despite its capacity it is surprisingly light (around 350g dry – a little heavier than Katadyn ‘s claimed 290g) and packs down to around the same size as a pair of lightweight overtrousers. It can easily be filled at a water source and then rolled up and sealed for carrying onwards to camp – I strapped it to the outside of my rucksack, Samwise Gamgee-style. The weight penalty of carrying a water filter can be offset by not needing to carry as much water with you at any one time, but inevitably you will still need more water at night for cooking.

It looks like it was designed primarily as a basecamp item, yet the Gravity is also light enough to make it worthwhile for multi-day backpacking trips or even out on a day walk as an alternative to carrying lots of drinking water with you from the start. Of course the water bag needs to be hung somewhere higher than the container you are using to collect the filtered water – which is great if you have a tree, but will require some creative thinking if you are in the mountains above the treeline. After some experimentation I found that a tripod of trekking poles would work, and even a tent peg wedged into a crack in a rockface. As a last resort you could just recruit a friend to hold it whilst you retrieved the water, or juggle the bag and nozzle carefully to do it solo.

The catch with most water filtration systems is that they are only as effective as your method for keeping the ‘drinking’ end clean. If you know that your water sources are potential sources of cryptosporidium or giardia then there is little point in spending several minutes filtering them out if you still have that contaminated water on the rim of the bottle when you put it to your mouth. This is of course true of the Katadyn system too, to an extent, but they do supply a separate Ziploc bag to keep everything separate in the storage bag. In addition you can always detach the hose via the quick release valve, to ensure that the drinking end stays uncontaminated. The flexibility of the hose means it can be tucked safely out of the way when not in use, and can be connected directly to some hydration bladders too.

The flexible drybag and the filter within seem rugged enough, but only within reason. It could be easily punctured if handled carelessly and would be very difficult to repair in the field, so it may be wise to carry a backup of chlorine tablets.